Image guidance for radiation therapy is an active area of investigation and technology development. Current radiotherapy practice utilizes highly conformal radiation portals that are directed at a precisely defined target region. It is desirable to provide an imaging method to assist in the placement of the radiation beam at the time of treatment. This technique is known as Image Guided Radiation Therapy (IGRT).
Commercially available techniques for IGRT typically use x-ray or ultrasound imaging technology to produce planar x-ray, computed tomography, or 3D ultrasound images. However, IGRT techniques based on x-rays or ultrasound are not ideally suited to IGRT. For example, x-rays suffer from low soft tissue contrast and are not ideally suited to imaging tumours. X-ray based techniques also use ionizing radiation and result in supplemental dose delivered to the patient. Ultrasound cannot be utilized in all locations of the body. Both x-ray and ultrasound based IGRT techniques are difficult to integrate with a linear accelerator such that they can provide real-time images in any imaging plane during treatment. Yet further, fiducial markers are used in conjunction with these imaging techniques. However, fiducial markers must be placed using an invasive technique, and are thus less desirable.
In order to overcome these deficiencies, it has been proposed to integrate a radiotherapy system with a Magnetic Resonance Imaging (MRI) device. For example, PCT Patent Application Publication No. WO 2007/045076 to Fallone et al., assigned to the assignee of the present application describes a medical linear accelerator (linac) that is combined with a bi-planar permanent magnet suitable for MRI.
An MRI device functions by providing a strong and homogeneous magnetic field that aligns the nuclear magnetic moments of target nuclei. For example, hydrogen nuclei (protons) are the most common imaging target in MRI. In the presence of the magnetic field, the magnetic moments of the nuclei align with the homogeneous magnetic field and oscillate at a frequency determined by the field strength, known as the Larmor frequency. This alignment can be perturbed using a radiofrequency (RF) pulse, such that the magnetization flips from being aligned with the direction of the magnetic field (B0 field) towards being perpendicular to the direction of the magnetic field and thus exhibits transverse magnetization. After the pulse, when the nuclei revert back to their aligned state, the transverse magnetic moment decays to zero, and the longitudinal magnetic moment increases to its original value. Different soft tissues exhibit different transverse and longitudinal relaxation times.
A specific magnetic field is applied across the patient utilizing gradient magnetic coils, and images of the patient can be formed by first generating a specific sequence of perturbing RF pulses and then analyzing the signals that are emitted by the nuclei as they return to their original magnetization state after being perturbed by the RF pulses. RF detector coils receive the emitted RF signals to provide MRI raw data.
However, the quality of an MRI image output by the MRI device may be affected by a pulsed treatment beam from the linac that is incident on the RF detector coils used to detect the RF signals that are generated while nuclei are relaxing after an exciting RF pulse. The incident radiation induces effects in the RF detector coils. An example of the radiation induced effects is a radiation induced current in the detector coil. Radiation induced current can interfere with the fidelity of imaging signals received by the detector coils. This problem manifests itself because, when irradiated with high-energy (megavoltage) photons, the high-energy electrons produced in Compton interactions are likely to escape the thin detector coil material, such as copper strips known to be used in MRI RF detector coils. If there is no influx of electrons to balance this effect, a net positive charge is created in the material. Therefore, if the coil material is part of an electrical circuit, a current induced by the radiation will begin to flow in order to neutralize this charge imbalance.
Since MRI imaging involves forming images based on current induced by RF signals in RF detector coils, radiation induced current in the MRI RF detector coils from an incident treatment beam can introduce artefacts thereby reducing the MRI signal to noise ratio (SNR). While it is possible to time the image acquisition process and the pulsing of radiation so that a radiation pulse is not emitted at the exact same time as the MRI detector coils are receiving RF signals for imaging, such a restriction can limit the adaptability of the system. It would be advantageous to be able to irradiate while imaging.
U.S. patent application Ser. No. 13/253,589 to Rathee et al., which is incorporated entirely herein by reference, is directed to a radiation therapy system comprising a radiation source of generating a beam of radiation, a magnetic resonance imaging (MRI) apparatus comprising at least one detector coil, and an electrically grounded dielectric material between the radiation source and the radiofrequency detector coil for shielding the at least one radiofrequency detector coil from the beam of radiation. Shielding the RF detector coil from the beam of radiation with an electrically grounded dielectric material significantly reduces the radiation induced current in the at least one radiofrequency detector coil, and therefore significantly reduces the amount of radiation induced noise in the MRI images due to radiation.
U.S. Patent Application Publication No. 2011/0087090 to Boernert et al. is directed to a radiation therapy system comprising a radiation therapy subsystem configured to perform radiation therapy by applying radiation pulses to a region of a subject at pulse intervals; a magnetic resonance (MR) imaging subsystem configured to acquire a dataset of MR imaging data samples from the region of the subject over one or more MR sampling intervals that are longer than the pulse intervals, the one or more MR sampling intervals overlapping at least some of the pulse intervals; a synchronizer configured to identify MR imaging data samples of the data set whose acquisition times overlap pulse intervals; and a reconstruction processor configured to reconstruct the dataset, without the measured values for the MR imaging data samples identified as having acquisition times overlapping pulse intervals, to generate a reconstructed MR image. The system requires a synchronizer to ascertain the radiation pulses that overlap with the MR sampling intervals in order to reconstruct the MR imaging data affected by the radiations pulses.
Accordingly, it is desired to provide an alternate method for reducing the deleterious effect of radiation induced current from a linac treatment beam incident on RF detector coils of an MRI device.